JP5975417B2 - Manufacturing method of light receiving element - Google Patents

Description

  The present invention relates to a light receiving element, a semiconductor epitaxial wafer, a manufacturing method thereof, and a detection apparatus that receive light in a near infrared region to a mid infrared region.

  Near-infrared to mid-infrared light corresponds to an absorption spectrum region related to living organisms such as animals and plants, and the environment. Therefore, near-infrared to mid-infrared light using a III-V compound semiconductor for the light receiving layer is used. Detectors are being developed. For example, a CMOS (Complementary Metal-Oxide Semiconductor), which is a read-out IC (ROIC), is connected to a light-receiving element array that has sensitivity up to a wavelength of 2.6 μm by using Extended-InGaAs for the light-receiving layer. An example of a detector that converts photocharge into an output signal has been published (Non-Patent Document 1). In the light receiving element array, InAsP that lattice matches with the InGaAs light receiving layer is used for the window layer.

In addition, a pin-type photodiode having an InGaAs / GaAsSb type 2 type multi-quantum well structure (MQW: Multi-Quantum Wells) in the light receiving layer and having a pixel region of p-type has sensitivity up to a wavelength of 2.5 μm. It has been reported (Non-Patent Document 2).
In addition, a light-receiving element has been proposed in which a light-receiving layer has an InAs / GaSb type 2 multiple quantum well (MQW) structure on a GaSb substrate (Non-patent Document 3). According to this light receiving element, it is shown that the sensitivity is close to a wavelength of 12 μm.
In addition, a light-receiving element having an nBn (n-type layer / barrier layer / n-type layer) structure in which (InAs / GaSb) type 2 MQW is used as a light-receiving layer on a GaSb substrate and a barrier layer is disposed in the middle of the light-receiving layer. It has been proposed (Non-Patent Document 4). Compared with the pin structure, the nBn structure uses hole diffusion for light detection, so that mesa etching for pixel separation is shallow, and there is an advantage that noise current flowing through the side wall of the mesa structure can be reduced.

Hideo Takahashi et al. “InGaAs photodetector for near infrared”, OPTRONICS (1997), No.3, pp.107-113 R.Sidhu, N.Duan, J.C.Campbell, and A.L.Holmes, Jr., "A 2.3μm cutoff wavelength photodiode on InP using lattice-matched GaInAs-GaAsSbtype II quantum wells” 2005 International Conference on Indium Phosphide and Related Materials Binh-Minh Nguyen, Darin Hoffman, Yajun Wei, Pierre-Yves Delaunay, Andrew Hood, and Manijeh Razeghi, `` Very high quantumefficiency in type-II InAs / GaSb superlatticephotodiode with cutoff of 12 μm ”Appl. Phys. Lett., Vol. 90, 231108 HSKim, E. Plis, JBRodriguez, GDBishop, YDSharma, LRDawson, S. Krishna, J. Bundas, R. Cook, D. Burrows, R. Dennis, K. Patnaude, A. Reisinger and M. Sundaram , “Mid-IR focal plane arraybased on type-II InAs / GaSbstrain layer superlattice detector with nBn design” Appl. Phys. Lett., Vol.92, 183502

However, the near-infrared InGaAs photodetector (image sensor) of Non-Patent Document 1 described above uses InGaAs having a composition that does not lattice match with the InP substrate as a light receiving layer, so that dark current increases and noise increases. Further, it is difficult for the detectable wavelength to exceed 2.6 μm.
In addition, with respect to the light receiving element of Non-Patent Document 2, there has been no report of an example that actually reaches a wavelength of 3 μm.
Regarding the light-receiving element of Non-Patent Document 3, the GaSb substrate is expensive, has a large variation in quality, and has a problem in terms of mass productivity. A particularly essential problem is that GaSb has absorption due to free carriers in the mid-infrared region, so that the sensitivity is lowered at the backside incidence of the substrate, which is inevitable in an arrayed pixel.
The light receiving element of Non-Patent Document 4 has the same problem as that of Non-Patent Document 3 described above, and encounters difficulty in terms of mass productivity.

  The present invention provides a light receiving element, a semiconductor epitaxial wafer used therein, a manufacturing method thereof, and a detection apparatus, which have high light receiving sensitivity in the near infrared region to the mid infrared region and can stably obtain high quality. Objective.

The light receiving element of the present invention is formed of a III-V group compound semiconductor. The light receiving element includes an InP substrate transparent to light having a wavelength of 3 μm to 12 μm, a buffer layer positioned in contact with the InP substrate, and a light receiving layer having a multiple quantum well structure positioned in contact with the buffer layer. The light receiving layer has a cutoff wavelength of 3 μm or more and is lattice-matched to the buffer layer. The buffer layer comprising a GaSb layer having a value of | a ₂ −a ₁ | / a ₁ exceeding the range of formal lattice matching conditions in a buffer layer having a lattice constant a ₂ and an InP substrate having a lattice constant a ₁ layer is directly epitaxially grown on the InP substrate. The buffer layer has a thickness of 1 μm or more, and the multiple quantum well structure includes a Ga element and an Sb element as constituent elements.

In the group III-V compound semiconductor, the lattice constant is changed by changing the composition of the element. Therefore, when the epitaxial layer is grown, the composition of the epitaxial layer is usually changed finely so as to match the lattice constant of the lower layer. As a result, a stacked body of III-V group compound semiconductors having a low lattice defect density is formed, and the dark current, which is one of the most important indices in the light receiving element, can be reduced. In order to obtain a certain level of characteristics, it is necessary to reduce the lattice defect density not only in the light receiving element but also in other semiconductor elements. Considering the importance of low lattice defect density, reflecting the circumstances peculiar to such III-V compound semiconductors, or not only III-V groups but also peculiar to compound semiconductors. In that case, the degree of lattice mismatch: | s ₁ −s ₂ | / (s ₂ or s ₁ ) represented by the lower layer lattice constant s ₁ and the upper layer lattice constant s ₂ grown thereon is as low as possible. For example, in the InGaAs / InP system, (0.002 to 0.003) or less is desirable. In general, it is known that the degree of lattice mismatch is 0.005 or less. Considering these, the range in which the degree of lattice mismatch is 0.005 or less can be regarded as the range of the above-mentioned formal lattice matching conditions. Exceeding the range of formal lattice matching conditions means that the degree of lattice mismatch exceeds 0.005.
However, since the limit value of the degree of lattice mismatch actually depends on the material system, the GaSb / InP system targeted by the present invention has a good crystallinity even if the degree of lattice mismatch is large. A stack of group compound semiconductors can be formed. Whether the upper layer is epitaxially grown with respect to the lower layer cannot be determined under the conventional lattice matching conditions that only incorporate the lattice constant. The present invention is an invention based on demonstration data in which such an example appears.
Here, the buffer layer is epitaxially grown without saying that it is lattice-matched to the InP substrate. The term lattice matching is close to the lattice constants of both the growth layer and the grown layer. In some cases it is implied to be in range, and the term epitaxial growth was used to avoid such misunderstandings. In other words, a case where growth is performed while maintaining “lattice matching” or “substantially lattice matching” at a low lattice defect density even though the lattice constants of both the growth layer and the layer to be grown are greatly different. .
According to the above configuration, a buffer layer that does not conform to the lattice matching condition can be formed using an InP substrate known for good crystallinity, and an MQW light-receiving layer lattice-matched on the buffer layer can be obtained. . Since the band gap of InP is 1.35 eV, the light-receiving layer having a cutoff wavelength of 3 μm does not absorb light in the wavelength range to be received. A band gap of 1.35 eV corresponds to a short wavelength of less than 1 μm. For this reason, in the light receiving element including the two-dimensionally arranged pixels in which the back surface incidence of the substrate is inevitable, the target light is not absorbed at all by the substrate, so that high light receiving sensitivity can be maintained. For MQW having a cutoff wavelength of 3 μm or more, even if it is Type 2 MQW, it is common to use a substrate of a III-V group compound semiconductor having a larger lattice constant than InP (lattice constant 5.869８６). . Even when the transition energy of electrons in light reception is reduced using Type 2 MQW, a substrate having a lattice constant larger than that of InP is required to realize a long wavelength (small transition energy) of a cutoff wavelength of 3 μm. A substrate having a larger lattice constant than the InP substrate absorbs near-infrared to mid-infrared light due to various factors including a large lattice constant. Since the substrate is thick, the absorption of the target light by the substrate causes a significant decrease in sensitivity.

  By inserting a buffer layer made of a GaSb layer as described above, a light receiving element having high sensitivity in the near infrared to mid infrared can be obtained by MQW formed on an InP substrate and having a cutoff wavelength of 3 μm or more. In addition, as described above, since the InP substrate has a stable and excellent crystallinity, even when the lattice defect density is inherited to the upper layer, an MQW having a good crystallinity can be stably obtained. Dark current can be reduced. In addition, the stable and good crystallinity of the InP substrate brings about uniformity of the light receiving element characteristics and an improvement in yield. Furthermore, since a large-diameter substrate can be obtained as compared with other III-V group compound semiconductor substrates, it is excellent in mass productivity.

Note that the MQW lattice constant is determined from the periodicity of an XRD (Xray Diffraction) pattern when the MQW is configured by repeating pairs of the a layer and the b layer. The lattice constant of the buffer layer is an average value over the entire thickness of the buffer layer having a predetermined thickness directly grown on the InP substrate, but is almost the same as the lattice constant specific to the material described in the handbook or the like.

The buffer layer is composed of a GaSb layer. Alternatively, it may be composed of a layer (hereinafter referred to as GaSb equivalent layer) having the same crystal characteristics as the GaSb layer in relation to the InP substrate. Here, in relation to the InP substrate, the crystal characteristics are the same as those of the GaSb layer (hereinafter referred to as GaSb equivalent layer) means that Sb is included and the lattice constant is almost the same as that of the GaSb layer, and the lattice matching with the InP substrate. Although it deviates from the conditions, it means that it is epitaxially grown in the above-mentioned meaning. And it means that the light receiving layer can be formed by lattice matching with the GaSb equivalent layer.
As a result, a light receiving sensitivity on the long wavelength side through a GaSb (lattice constant a ₂ = 6.095Å) layer having a larger lattice constant than InP (lattice constant a ₁ = 5.869Å) or a GaSb equivalent layer. The light receiving layer can be epitaxially grown. The thickness of the buffer layer is not particularly limited, but is preferably 0.2 μm or more. If the thickness of the buffer layer is not 0.2 μm or more, the following problem occurs. That is, when the ground electrode is provided in the buffer layer, it is necessary to stop etching in the buffer layer when the epitaxial layer is etched from the upper layer to the predetermined thickness of the buffer layer. If the thickness of the buffer layer is 0.2 μm or more, it will not be within the variation range of etching stop. Also, the thicker one can be made to be 1 μm or more, preferably 1.5 μm or more, or 2 μm or more, since the thicker within the predetermined range, the better the crystallinity of the surface layer.
Although described in detail in the embodiment, the thickness of the buffer layer of 0.2 μm or more greatly exceeds the critical film thickness in terms of the relationship with the critical film thickness in the InP / GaSb. When the degree of lattice mismatch is 0.038, the critical film thickness is estimated to be about 4 nm (0.004 μm). Therefore, the thickness of the buffer layer can be said to be several tens of times the critical thickness. The GaSb buffer layer has a thickness of at least 50 times the critical film thickness. It may be 100 times or more. This is considered to be related to the specificity of the Sb element such as the surfactant effect.
From the standpoint of lattice matching, the above InP substrate / GaSb layer or GaSb equivalent buffer layer can be regarded as a substitute for the GaSb substrate. When a GaSb substrate is used, however, GaSb absorbs the target light of the light receiving layer because absorption by free carriers exists in the mid-infrared region. Since the substrate is thick, the absorption of the target light by the GaSb substrate causes a significant reduction in sensitivity. Since the InP substrate does not absorb light having a wavelength of 3 μm or more as described above, good sensitivity can be maintained.
Further, the InP substrate has stable and good crystallinity as compared with the GaSb substrate, and the characteristics of the light receiving element can be made uniform and the yield can be improved. Further, the InP substrate has a larger diameter than the GaSb substrate, and is excellent in mass productivity. Furthermore, since it is very inexpensive, it is possible to provide a high-quality light-receiving element that is excellent in economic efficiency.

An InP substrate that is transparent to light having a wavelength of 3 μm to 12 μm is preferably an InP substrate to which sulfur (S) is not added.
An InP substrate containing sulfur (S) has a transmittance that decreases from a wavelength of 3 μm or more, and approaches almost zero at a wavelength of 5 μm, and becomes zero at a wavelength of 5 μm or more. For this reason, an InP substrate that does not contain sulfur must be used.

As an InP substrate transparent to light having a wavelength of 3 μm to 12 μm, it is particularly preferable to use an Fe-containing InP substrate or a non-doped InP substrate.
By using the Fe-containing InP substrate or the non-doped InP substrate, the InP substrate becomes transparent to light having a wavelength of 3 μm to 12 μm, and the sensitivity of the light receiving element of the present invention that receives light in this wavelength region as a light receiving target can be increased.

A pn junction can be provided in the light receiving layer.
With the above configuration, it is possible to provide an infrared light receiving element having a pin structure and high light receiving sensitivity.

Alternatively, the light receiving layer has an insertion layer made of a group III-V compound semiconductor lattice-matched with the light receiving layer, and the bottom of the conduction band of the insertion layer is higher than the bottom of the conduction band of the light receiving layer It can be.
Accordingly, an infrared light receiving element having an nBn (n-type layer / barrier layer / n-type layer) structure having high light receiving sensitivity can be provided. Further, the leakage current can be reduced while ensuring the independence of the pixels.

The multiple quantum well structure in the light receiving layer of the light receiving element may be any of type 2 MQW, {(InAs / GaSb), (InAs / InGaSb), (InAsSb / GaSb), and (InAsSb / InGaSb)}.
Thereby, the light receiving element can obtain a light receiving layer having sensitivity in the near infrared to mid infrared (wavelength 3 μm to 12 μm). Note that the lattice constant of InAs is 6.058６, the lattice constant of GaSb is 6.095Å, the lattice constant of In _0.2 Ga _0.8 Sb is 6.172Å, and the lattice constant of InAs _0.92 Sb _0.08 is 6.092Å. .

It can have a structure in which light is incident from the back surface of the InP substrate.
By using the back surface of the InP substrate as the incident surface, two-dimensionally arranged light receiving elements (pixels) are connected to the readout electrodes of the readout circuit (ROIC) with a compact structure by the micro bump connection method, and the hybrid detection is miniaturized. A device can be obtained. Rather, a hybrid detection device that is small and easy to use cannot be obtained unless the micro bump connection method is used. In addition, since the InP substrate does not absorb near-infrared to mid-infrared light, a light receiving element having high sensitivity in the wavelength range can be obtained.
Here, the structure in which light is incident from the substrate side corresponds to an anti-reflection film (AR (Anti-reflection) film, etc.) provided on the back surface of the substrate, and two-dimensionally arranged pixels (light receiving elements) themselves. Since the backside incidence of the substrate is also planned, it corresponds to this structure.

The detection device of the present invention includes any one of the light receiving elements described above and a readout circuit (ROIC: Read Out IC), and the pixel electrode in the light receiving element and the readout electrode in the readout circuit are connected via a bump. It is characterized by.
As a result, a compact and downsized detection device having high light receiving sensitivity in the near infrared to mid infrared can be obtained.

The semiconductor epitaxial wafer of the present invention is formed of a III-V group compound semiconductor. This semiconductor epitaxial wafer includes an InP substrate that is transparent to light having a wavelength of 3 μm to 12 μm, and a buffer layer that is positioned in contact with the InP substrate. In the buffer layer having the lattice constant a ₂ and the InP substrate having the lattice constant a ₁ , the value of | a ₂ −a ₁ | / a ₁ exceeds the range of the formal lattice matching condition. And a GaSb layer.

Regarding the epitaxial growth in the III-V group compound semiconductor and the formal lattice matching conditions, the description in the light receiving element is applied as it is. The technical meaning of configuring the buffer layer with a GaSb layer is also as described above. The technical meaning of forming the GaSb equivalent layer is also as described above.
With the above structure, a buffer layer having a lattice constant that is not lattice-matched with InP is formed using a GaSb layer or the like by using an InP substrate that is known for good crystallinity. Can be good. An epitaxial layer lattice-matched to the buffer layer can be grown on the semiconductor epitaxial wafer. In other words, a substrate having a lattice constant different from that of InP can be realized while using an InP substrate.

In the above epitaxial wafer, an InP substrate transparent to light having a wavelength of 3 μm to 12 μm can be an Fe-containing InP substrate or a non-doped InP substrate.
Accordingly, since the substrate having a large thickness does not absorb light in the wavelength range of 3 μm to 12 μm, the sensitivity of the light receiving element according to the present invention that receives light in this wavelength range can be increased.

The epitaxial wafer may include a first semiconductor layer including a multiple quantum well structure that is lattice-matched to the buffer layer.
With the above configuration, the first semiconductor layer including a multiple quantum well structure with good crystallinity can be grown while being lattice-matched. Therefore, it is possible to obtain an infrared light receiving element with high light receiving sensitivity using an InP substrate having a high transmittance.

A multiple quantum well structure included in the first semiconductor layer of the semiconductor epitaxial wafer is converted into a type 2 multiple quantum well structure, {(InAs / GaSb), (InAs / InGaSb), (InAsSb / GaSb), and (InAsSb / InGaSb)}. It can be either.
Thereby, it is possible to provide a semiconductor epitaxial wafer for producing a light receiving element having high sensitivity in the near infrared to mid infrared (wavelength 3 μm to 12 μm).

The method for manufacturing a light receiving element of the present invention manufactures a light receiving element in which a group III-V compound semiconductor is directly laminated on an InP substrate. This manufacturing method includes a step of preparing an InP substrate transparent to light having a wavelength of 3 μm to 12 μm, a step of forming a buffer layer in contact with the InP substrate, and a lattice matching with the buffer layer in contact with the buffer layer. Forming a light receiving layer including a multiple quantum well structure having a wavelength of 3 μm or more. Then, in the buffer layer forming step, the buffer layer is epitaxially grown from the InP substrate while the lattice constant a ₂ of the buffer layer and the lattice constant a _{1 of} the InP substrate exceed the range of formal lattice matching conditions, And a GaSb layer having a thickness of 1 μm or more.

  According to the above method, by using a GaSb layer or the like, which is a material that is not lattice matched to the InP substrate, as the buffer layer, the buffer layer can have a crystallinity that is sufficiently crystalline so that MQW can be epitaxially grown on the InP substrate. it can. As a result, a light receiving element having a light receiving sensitivity as high as 3 μm or more can be obtained for the above reason. Further, the InP substrate can be stably obtained with a large diameter having good crystallinity, and a light receiving element with high quality and high cost can be provided. The buffer layer may be formed of a GaSb equivalent layer as described above.

In the method for manufacturing a light receiving element, the multiple quantum well structure included in the light receiving layer is changed to a type 2 multiple quantum well structure, {(InAs / GaSb), (InAs / InGaSb), (InAsSb / GaSb), and (InAsSb / InGaSb). }.
Accordingly, a light receiving element having high sensitivity in the near infrared to mid infrared (wavelength 3 μm to 12 μm) can be easily manufactured.

In the light receiving layer forming step, a pn junction can be formed in the light receiving layer.
With the above configuration, it is possible to provide an infrared light receiving element having a pin structure and high light receiving sensitivity.

In the light-receiving layer forming step, a lattice-matched insertion layer made of a III-V group compound semiconductor lattice-matched to the light-receiving layer is inserted into the light-receiving layer, and the bottom of the conduction band of the insertion layer is It can be higher than the bottom of the conduction band of the light receiving layer.
As a result, an infrared light-receiving element having an nBn (n-type layer / barrier layer / n-type layer) structure having high light receiving sensitivity is produced while reducing the leakage current by reducing the depth of the mesa structure groove. be able to.

The method for producing a semiconductor epitaxial wafer of the present invention produces a semiconductor epitaxial wafer of a III-V compound semiconductor on an InP substrate. This manufacturing method includes a step of preparing a transparent InP substrate to light of a wavelength 3Myuemu～12myuemu, Ru comprising the step of forming a buffer layer in contact with the InP substrate. In the step of forming the buffer layer, lattice constants a ₂ and a lattice constant a ₁ of the InP substrate of the buffer layer, but formal consisting GaSb layer is beyond the scope of lattice matching conditions the buffer layer on the InP substrate And in the step of forming the light receiving layer, the light receiving layer including a multiple quantum well structure is epitaxially grown on the buffer layer , and the buffer layer has a thickness of 1 μm or more, The multiple quantum well structure includes a Ga element and an Sb element as constituent elements .

  By using the InP substrate known for good crystallinity by the above method and forming a buffer layer with GaSb or the like, which is a material having a lattice constant significantly different from that of InP, it is relatively A good crystalline buffer layer can be obtained. The thickness of the buffer layer is not particularly limited, but may be, for example, 1 μm or more, may be 1.5 μm or more, and may be 2 μm or more. An epitaxial layer lattice-matched to the buffer layer can be grown on the semiconductor epitaxial wafer. The buffer layer may be formed of a GaSb equivalent layer as described above.

  According to the light receiving element or the like of the present invention, the InP substrate is used while obtaining a light receiving layer having a cutoff wavelength of 3 μm or more, and thus the substrate does not absorb light in the target wavelength region. Therefore, it is possible to obtain a light receiving element with high sensitivity in the wavelength range of 3 μm or more. In addition, since the InP substrate has stable and good crystallinity, a high-quality light receiving element or the like can be obtained stably. In addition, since a large-diameter InP substrate can be obtained, it is excellent in mass productivity.

2A and 2B illustrate a light receiving element according to Embodiment 1 of the present invention, in which FIG. 1A illustrates a light receiving element including two-dimensionally arranged pixels, and FIG. 2B illustrates a single pixel light receiving element. It is a figure which shows the transmittance | permeability of an InP board | substrate. It is a figure which shows the transmittance | permeability of a GaSb board | substrate. It is sectional drawing which shows the hybrid detection apparatus which combined the light receiving element and read-out circuit of Fig.1 (a). It is a figure which shows the semiconductor epitaxial wafer of this invention. It is a flowchart of the manufacturing method of the light receiving element shown in FIG. It is sectional drawing which shows the light receiving element in Embodiment 2 of this invention. Embodiment 3 of the present invention is shown, in which (a) is an epitaxial wafer and (b) is a light receiving element. It is a figure which shows the result of XRD of the semiconductor epitaxial wafer of the example of this invention in an Example.

(Embodiment 1)
1A and 1B are diagrams showing a light receiving element 10 according to Embodiment 1 of the present invention, in which FIG. 1A shows a light receiving element in which pixels are two-dimensionally arranged, and FIG. 1B shows a single pixel light receiving element. is there. Both are the light receiving elements of the present invention, but the description of the light receiving element having the two-dimensionally arranged pixels includes the description of the single pixel light receiving element. The element will be described.
According to FIG. 1A, the light receiving element 10 has the following group III-V semiconductor stacked structure.
<InP substrate 1 / p-type GaSb buffer layer 2 / type 2 (InAs / GaSb) MQW / n-type contact layer 5>
Among these, the type 2 (InAs / GaSb) MQW is the light-receiving layer 3. It has a cutoff wavelength of 3 μm or more, and has light receiving sensitivity to light in the near infrared to mid infrared (for example, wavelength 3 μm to 12 μm). This MQW is preferably formed, for example, by a single (InAs / GaSb) as one pair and about 100 to 300 pairs. The thickness of InAs and GaSb is preferably in the range of 2 nm to 7 nm, for example, about 5 nm. Of the entire MQW, several tens of pairs of GaSb on the InP substrate 1 side are preferably doped with a p-type impurity such as Be. In addition, InAs is preferably doped with an n-type impurity such as Si so that several tens of pairs of the MQWs on the contact layer 5 side of the entire MQW are formed as n-type layers. Both intermediate layers are not doped with impurities to be i (intrinsic) type. By forming the conductive type region in the MQW, a pin photodiode can be obtained.
A pn junction or a pin junction is formed in the MQW 3 by the impurity doping or undoping described above.
The electrode 11 of the pixel P is preferably formed of an AuGeNi alloy or the like so as to make ohmic contact with the n-type contact layer 5. The ground electrode 12 is preferably formed of a Ti / Pt / Au alloy or the like so as to make ohmic contact with the p-type GaSb buffer layer 2. At this time, since an electrode is formed in the buffer layer, the buffer layer preferably has a carrier concentration of 1E18 cm ⁻³ or more.
Light enters from the back surface of the InP substrate 1. An AR (Anti-reflection) film 35 covers the back surface of the InP substrate 1 in order to prevent reflection of incident light. The structure in which the AR film 35 is disposed on the back surface of the InP substrate 1 can be said to be a structure for entering from the substrate side. Further, since the two-dimensional array of pixels P itself is a micro-bump connection method used for connection with a readout circuit, substrate side incidence is inevitable, and it can be said that the structure is incident from the above substrate side.

The feature in the present embodiment is as follows.
(1) a lattice constant _a 1 = _5.869Å of the InP substrate 1, since the lattice constant _a 2 = _6.095Å of _{GaSb, | a 2 -a 1 |} / a 1 = 0.038 (3.8 %). Although the difference in lattice constant between the lower layer and the upper layer is very large, the GaSb buffer layer 2 has relatively good crystallinity and is epitaxially grown on the InP substrate. The reason for this is not clear at this time. Based on the past research results, it is considered that such good crystallinity is related to the thickness of the GaSb buffer layer and the specificity of the Sb element such as the surfactant effect. It is done.
In general, the greater the degree of lattice mismatch, the more misfit dislocations increase unless the film thickness of the layer to be grown is reduced, and an epitaxial film with good crystallinity cannot be obtained. For this reason, in the field of crystal growth, the concept of a critical film thickness is generally provided, and the idea that an epitaxial film with good crystallinity cannot be obtained beyond the critical film thickness. The critical film thickness is determined by the Matthews and Blakeslee equation (for example, A. Braun et.al. Journal of Crystal Growth 241 (2002) 231-234) according to the mechanical equilibrium theory or the People and Bean equation according to the energy equilibrium theory. Desired. A diagram showing the relationship between the critical film thickness and the degree of lattice mismatch based on these equations is shown in “Masayoshi Umeno, Tetsuo Soga: Crystal Growth Handbook (Kematsu Keisho, Kyoritsu Shuppan, 1995) p. 699”. Has been. InP / GaSb has a lattice mismatch of 0.038 as described above. According to the above-mentioned crystal growth handbook, in this case, it is about 4 nm (0.004 μm) according to the People and Bean equation and about 1 nm (0.001 μm) according to the Matthews and Blakeslee equation.
As described above, the thickness of the GaSb buffer layer 2 is preferably 0.2 μm or more. Therefore, in the present embodiment, the thickness of the GaSb buffer layer 2 is several tens of times the critical film thickness. Even based on approximately 4 nm according to the People and Bean equation for calculating the critical film thickness, the thickness is 50 times or more. The thickness may be 100 times or more.
The GaSb buffer layer 2 grown on the InP substrate 1 has a mirror surface and is flat without unevenness. The FWHM (full width at half maximum) of the main diffraction peak of XRD is preferably 300 seconds or less. The carrier concentration in the p-type GaSb buffer layer 2 is 1E18 cm ⁻³ or more in order to reliably realize the ohmic contact.
(2) The InP substrate 1 has a band gap energy of 1.35 eV. This band gap corresponds to a wavelength of less than 1 μm. For this reason, the light receiving layer 3 does not absorb the target light. FIG. 2 is a diagram showing the results of measuring the transmittance (room temperature) of an InP substrate having a thickness of 350 μm with a Fourier transform infrared spectrophotometer (FT-IR). The Fe-doped (high resistance) InP substrate is transparent in the wavelength range of 2 μm to 12 μm and has no absorption band. FIG. 2 also shows the transmittance of the sulfur-doped InP substrate, but the transmittance is almost zero at a wavelength of 5 μm or more. The low transmittance at a wavelength of less than 5 μm is due to the effect of rough back polishing. Since the transmittance is zero at a wavelength of 5 μm or more, the sulfur-containing InP substrate cannot be used as a substrate of an infrared light receiving element targeted by the present invention.
When forming a type 2 MQW having a cutoff wavelength of 3 μm or more, it is common to use a group III-V compound semiconductor having a lattice constant larger than that of InP as the material constituting the MQW layer. If a group III-V compound semiconductor lattice-matched to InP is used, a cut-off wavelength of 3 μm or more cannot be realized even if the difference in transition energy of electrons during light reception is reduced by type 2 MQW.
When realizing a cutoff wavelength of 3 μm or more with a type 2 MQW, for example, a GaSb substrate is usually used. Such a group III-V compound semiconductor having a large lattice constant often has an absorption band in the near-infrared to mid-infrared wavelength region. For example, GaSb has absorption by free carriers in the mid-infrared region as shown in FIG. FIG. 3 is a diagram showing the results of measuring the transmittance (room temperature) of a GaSb substrate having a thickness of 500 μm by FT-IR. According to FIG. 3, the undoped GaSb substrate has a transmittance of almost zero at a wavelength of 5 μm or more. Also, the Te-doped GaSb substrate, which has a slightly higher transmittance than the undoped GaSb substrate, gradually decreases in transmittance from about 70% near the wavelength of 5 μm to about 50% near the wavelength of 6.5 μm, and the wavelength of 8 μm. It has fallen to 25% or less by the above. It is difficult to use a GaSb substrate exhibiting such a transmittance as a light-receiving element for the infrared region. For example, after manufacturing an epitaxial wafer, it is necessary to delete the GaSb substrate or greatly reduce the thickness, which leads to an increase in man-hours and a deterioration in quality.
However, according to the above, the GaSb buffer layer 2 having good crystallinity can be grown on an InP substrate having no absorption band in the near infrared to mid infrared, and the type 2 having a cutoff wavelength of 3 μm or more is formed on the GaSb buffer layer 2. (InAs / GaSb) MQW is formed. As a result, the InP substrate 1 having a large thickness does not absorb the target light. As a result, the sensitivity to the light to be received can be improved.
(3) Compared with a GaSb substrate, an InP substrate having good crystallinity can be obtained stably. For this reason, any portion of the InP substrate 1 / GaSb buffer layer 2 / (InAs / GaSb) MQW light-receiving layer 3 and the light-receiving layer 3 made of (InAs / GaSb) MQW having good crystallinity even when epitaxially grown. But you can get it on any occasion. As a result, elements with uniform characteristics can be manufactured with high yield.
Furthermore, since the InP substrate can be obtained with a larger diameter than the GaSb substrate, it is excellent in mass productivity. Furthermore, since the InP substrate is less expensive than the GaSb substrate, the light receiving element 10 and thus the detection device 50 that are excellent in economic efficiency can be provided.

FIG. 4 is a diagram showing a hybrid detection apparatus in which the light receiving element 10 of FIG. 1 and a readout circuit 70 formed on silicon (Si) are connected. The read circuit 70 is a CMOS (Complimentary Metal Oxide Semiconductor). The pixel electrode 11 that is conductively connected to the n-type contact layer 5 is conductively connected to the readout electrode 71 with the bumps 9 and 79 interposed therebetween. The ground electrode 12 conductively connected to the p-type buffer layer 2 has the wiring electrode 12e transmitted along the protective film 43 so as to be aligned with the height of the pixel electrode 11, and a CMOS ground electrode 72 and bumps interposed therebetween. Conductive connection.
According to the connection of the micro bump connection system with the bumps interposed as described above, a compact and downsized detection device can be obtained even if the pixel pitch is reduced and the pixels are high density.

FIG. 5 is a plan view showing the semiconductor epitaxial wafer 1a in a process in the middle of manufacturing the light receiving element 10 of FIG. Depending on the specifications of the light receiving element 10, in the case of an 8.5 mm × 10 mm chip (light receiving element) in which about 80,000 pixels are arranged vertically and horizontally at a pitch of 30 μm, about 11 pieces from a 2 inch diameter InP substrate, Can be acquired. About 52 pieces can be obtained from a 4-inch diameter InP substrate. As described above, higher mass productivity can be obtained by using a large-diameter InP substrate than using a GaSb substrate.
The semiconductor epitaxial wafer 1a shown in FIG. 5 is in a state where a GaSb buffer layer is grown on an InP substrate. The GaSb buffer layer has a thickness of 1 μm or more, exhibits a mirror surface, and is flat without unevenness. And the FWHM of the main peak by XRD is 300 seconds or less. However, | a ₂ −a ₁ | / a ₁ = about 0.038 (3.8%). Such good crystallinity is considered to be derived from the fact that the GaSb buffer layer is thick.
The light receiving element 10 is separated from the semiconductor epitaxial wafer 1a in a state in which the light receiving layer 3, the contact layer 5, the mesa structure, and the electrodes 11 and 12 are formed and the outline of the chip becomes gradually clear. FIG. 5 is a diagram of the stage where the GaSb buffer layer 2 is formed.

FIG. 6 is a flowchart showing a method for manufacturing the light receiving element 10 of FIG. First, an InP substrate is prepared and cleaned, and then a GaSb buffer layer 2 is grown to a thickness of 1 μm or more. The growth method is not particularly limited, and MBE (Molecular Beam Epitaxy) method, MOVPE (Metal Organic Vapor Phase Epitaxy) method and the like can be used. After growing the buffer layer, the type 2 (InAs / GaSb) MQW light-receiving layer 3 is grown. Since type 2 transition (light reception) is performed across the interface between InAs and GaSb, the greater the number of interfaces, the higher the light reception sensitivity on the long wavelength side. Therefore, when the sensitivity on the long wavelength side is important, the MQW is preferably about 150 pairs or more as a whole. In order to form a pn junction in the light receiving layer 3, that is, the MQW described above, p-type impurity Be is doped into about 50 MQW GaSb near the InP substrate 1 during MQW growth. The MQW formed thereafter is an intrinsic semiconductor (i-type: intrinsic) as undoped. Thereafter, n-type impurity Si is doped into the final InAs of about 50 MQW pairs. As a result, a pin-type or nip-type photodiode can be obtained. The pin junction is also a kind of pn junction. In addition, depending on the structure, there may be only a pi junction in the light receiving layer, but when considering the region where the electrode contacts, a pin junction is obtained. This can also be interpreted as a pn junction located in the light receiving layer.
Next, a mesa structure in which a trench is inserted between the pixels P is formed by etching. Etching is performed by wet etching with phosphoric acid + hydrogen peroxide water + water, or dry etching with hydrogen iodide or silicon chloride gas. As a result, each pixel P can prevent crosstalk and the like independently of surrounding pixels. Next, as shown in FIG. 1, the surface is covered with a protective film (passivation film) 43 that protects the surface of the mesa structure. As the protective film (passivation film) 43, an SiO ₂ film or the like is preferably used.
Thereafter, the pixel electrode 11 and the ground electrode 12 are formed by photolithography.

The light receiving element 10 includes the light receiving layer 3 of type 2 (InAs / GaSb) MQW having a cutoff wavelength of 3 μm or more on the GaSb buffer layer 2 on the InP substrate 1, so that even in the case of substrate back-side incidence, Since the InP substrate 1 having a large thickness does not absorb light of the target wavelength, the light receiving sensitivity can be increased.
In addition, by using an InP substrate having excellent crystallinity, a reliable reason is unknown, but a type 2 (InAs / GaSb) MQW light-receiving layer 3 having a low lattice defect density can be obtained. Dark current can be reduced.
Furthermore, since the InP substrate is less expensive than GaSb, it is possible to provide a light receiving element and a detection device that are excellent in economic efficiency.

(Embodiment 2)
FIG. 7 is a cross-sectional view showing the light receiving element 10 according to the second embodiment of the present invention. In the light receiving element 10, the pixel P includes an n-type region 6 selectively diffused from the opening of the SiN selective diffusion mask pattern 36 and a pn junction 15 positioned at the tip of the n-type region 6 as main parts. The pn junction 15 reaches the light receiving layer 3. Further, as described above, a pi junction may be used. Each pixel P is separated from surrounding pixels by a region that is not selectively diffused.
The structure of the III-V compound semiconductor stack is <InP substrate 1 / p-type GaSb buffer layer 2 / type 2 (InAs / GaSb) MQW light-receiving layer 3 / p-type contact layer 55>. In the present embodiment, the region in contact with the pixel electrode 11 is the n-type region 6 in which n-type impurities are selectively diffused. Each pixel P is separated by a region that is not selectively diffused, and the crystal layer remains as it is. For this reason, since the side walls of the pixels are not exposed unlike the mesa structure, the crystal is hardly damaged. As a result, it is easy to realize a low dark current.
For other configurations and operations, the description of the first embodiment is applied as it is.

(Embodiment 3)
FIG. 8A is a cross-sectional view showing the epitaxial wafer in the third embodiment of the present invention, and FIG. 8B is a cross-sectional view showing the light receiving element 10 in the third embodiment of the present invention. The epitaxial wafer shown in FIG. 8A has the following laminated structure.
<Fe-doped InP (100) substrate 101 / Te-doped GaSb buffer layer 2 / (InAs / GaSb) n ⁺ -type MQW21 light-receiving layer / (InAs / GaSb) n-type MQW22 / upper and lower MQW Also, a contact layer made of n ⁺ type MQW24 of barrier layer 23 / (InAs / GaSb) having a high bottom of the conduction band>
Of the n ⁺ -type MQW 21 and n-type MQW 22 that form the light-receiving layer, tens of pairs on the side in contact with the buffer layer 2, for example, 60 pairs of n ⁺ -type MQW 21 have an n-type carrier concentration of 2e18 cm ⁻³ or more. In this case, InAs is doped with n-type impurity silicon (Si) and GaSb is not doped. The n-type MQW 22 of (InAs / GaSb) positioned on the n ⁺ -type MQW 21 is, for example, 100 pairs and has an n-type carrier concentration of 1E16 cm ⁻³ . For the barrier layer 23, AlGaSb, AlAsSb, or the like having a wide band gap and having a conduction band bottom that is higher than the bottom of the upper and lower MQW conduction bands can be used. A single layer is usually used, but MQW may be used. The contact layer in which the pixel electrode 11 is to be arranged is, for example, 20 pairs, and is an n ⁺ type MQW 24 having a carrier concentration of 2e18 cm ⁻³ or more.

In the light receiving element 10 shown in FIG. 8B, the pixel P is separated from the peripheral portion by mesa etching only the contact layer of the n ⁺ -type MQW 24 or a part of the barrier layer. Although not shown, when pixels are arranged in an array, only the contact layer of the n ⁺ -type MQW 24 is mesa-etched to be separated from the adjacent pixels. The pixel electrode 11 is formed of a Ti / Pt / Au alloy or the like so as to make ohmic contact with the n type contact layer of the n ⁺ type MQW 24. The ground electrode 12 is preferably made of Ti / Pt / Au alloy or the like so as to make ohmic contact with the n ⁺ type MQW 21 positioned in contact with the buffer layer 2. In the present embodiment, both the pixel electrode 11 and the ground electrode 12 are n-side electrodes.
In the light receiving element 10 in the present embodiment, light reception is detected by capturing holes that diffuse and reach the pixel electrode among the electron-hole pairs generated by light reception. Since the movement of the holes to the pixel electrode is due to diffusion, the mesa structure groove for pixel separation can be shallowed. As a result, the noise current flowing through the side wall of the mesa structure can be kept low.
In order to configure the detection device, the surface is covered with an AR film, a protective film, or the like, as in the first embodiment. Then, the pixel electrode 11 and a readout electrode of a readout circuit (ROIC: Read-out IC) are conductively connected through a bump. The ground electrodes are also conductively connected.
In this embodiment, the advantages of providing the GaSb buffer layer 2 on the InP substrate are the same as those in the first and second embodiments.

The semiconductor epitaxial wafer 1a shown in FIG. 5 was produced and verified. The method for producing the semiconductor epitaxial wafer of the example of the present invention is as follows.
<Production method>:
A GaSb buffer layer was grown on the InP substrate by MBE. A GaSb buffer layer was grown to a thickness of 2 μm at a substrate temperature of 400 ° C. with a V / III ratio of 3.9. The growth rate was 1.1 μm / hour (about 1 ML / second). The semiconductor epitaxial wafer produced by the above method was observed with a metal microscope and XRD measurement.

<Observation with metal microscope>:
The surface of the GaSb buffer layer was a mirror surface. And it was smooth. Furthermore, no minute irregularities were observed.
<XRD>:
The results are shown in FIG. In FIG. 9, the main peak of the GaSb buffer layer is obtained together with the sharp main peak (100) of the InP substrate. The main peak of the GaSb buffer layer is not as good as that of the InP substrate, but the FWHM, which is an index of crystallinity, is 218.5 seconds. It can be said that the crystallinity is good. Since the main diffraction peaks of the InP substrate and GaSb were obtained with the entire surface of the InP substrate covered with the GaSb buffer layer, the main diffraction peak of the GaSb buffer layer is considered to be derived from the entire thickness of the GaSb buffer layer.
From the diffraction angle difference Δω between the main peak of the InP substrate and the main peak of the GaSb buffer layer, | a ₂ −a ₁ | / a ₁ = Δa / a was calculated, which was 0.0382. It is usually difficult to obtain a GaSb buffer layer having a mirror surface between the lower layer (InP substrate) and the upper layer (GaSb buffer layer) having such a large lattice constant difference. It is considered that increasing the thickness of the buffer layer has a positive effect.

  According to the semiconductor epitaxial wafer 1a, a type 2 MQW light-receiving layer having a large lattice constant can be grown in contact with a GaSb buffer layer having good crystallinity. This light receiving layer has a cutoff wavelength of 3 μm or more, and the InP substrate does not absorb light in the wavelength region. On the other hand, when a GaSb substrate is used, it has an absorption band due to free carriers in the wavelength range. Since the substrate is thick, this absorption causes a serious reduction in sensitivity. By using an InP substrate as in the example of the present invention, it is possible to avoid the problem of absorption by the substrate and maintain high sensitivity at a wavelength of 3 μm or more.

  Although the embodiments and examples of the present invention have been described above, the embodiments and examples of the present invention disclosed above are merely examples, and the scope of the present invention is the implementation of these inventions. It is not limited to the form. The scope of the present invention is indicated by the description of the scope of claims, and further includes meanings equivalent to the description of the scope of claims and all modifications within the scope.

  According to the light receiving element and the like of the present invention, high light receiving sensitivity can be maintained over the near infrared region to the mid infrared region. In addition, since an InP substrate having stable quality, large diameter, and economy is used for the substrate, a high-quality light receiving element or the like can be provided at low cost.

1 InP substrate, 2 GaSb buffer layer, 3 light-receiving layer, 5 n-type contact layer, 6 n-type region, 9 light-receiving element bump, 10 light-receiving element, 11 pixel electrode, 12 ground electrode, 12e wiring electrode, 15 pn junction, 21 n ⁺ type MQW, 22 n type MQW, 23 barrier layer, 24 n ⁺ type MQW, 35 antireflection film, 36 selective diffusion mask pattern, 43 protective film, 50 hybrid detector, 55 p type contact layer, 70 readout circuit , 71 readout electrode, 72 ground electrode, 73 insulating film, 79 bump, 101 Fe-doped InP (100) substrate, P pixel.

Claims (6)

A manufacturing method of a light receiving element in which a group III-V compound semiconductor is laminated,
Preparing a transparent InP substrate for light having a wavelength of 3 μm to 12 μm;
Forming a buffer layer in contact with the InP substrate;
Forming a light receiving layer including a multiple quantum well structure having a cutoff wavelength of 3 μm or more, in contact with the buffer layer and lattice-matching to the buffer layer,
In the buffer layer forming step, the buffer layer made of a GaSb layer in which the lattice constant a _{2 of the} buffer layer and the lattice constant a _{1 of} the InP substrate exceed the range of formal lattice matching conditions is formed on the InP substrate . with is directly epitaxial growth,
In the step of forming the light receiving layer, the light receiving layer including the multiple quantum well structure is epitaxially grown on the buffer layer,
The buffer layer may have a thickness of at least 1 [mu] m,
The method for manufacturing a light receiving element, wherein the multiple quantum well structure includes a Ga element and an Sb element as constituent elements .   The method for manufacturing a light receiving element according to claim 1, wherein the InP substrate transparent to light having a wavelength of 3 μm to 12 μm is an InP substrate to which sulfur (S) is not added.   The method for manufacturing a light receiving element according to claim 1, wherein the InP substrate transparent to light having a wavelength of 3 μm to 12 μm is a Fe-containing InP substrate or a non-doped InP substrate.   The multiple quantum well structure is any one of a type 2 multiple quantum well structure, {(InAs / GaSb), (InAs / InGaSb), (InAsSb / GaSb) and (InAsSb / InGaSb)}. The manufacturing method of the light receiving element of any one of Claims 1-3.   5. The method for manufacturing a light receiving element according to claim 1, wherein, in the step of forming the light receiving layer, a pn junction is formed in the light receiving layer. In the step of forming the light receiving layer, an insertion layer made of a group III-V compound semiconductor lattice-matched to the light receiving layer is inserted into the light receiving layer, and the bottom of the conduction band of the insertion layer is conducted to the conduction of the light receiving layer. The method for manufacturing a light receiving element according to claim 1, wherein the light receiving element is higher than a bottom of the belt.

Images